At present, the MEMS microphone available on the market is usually a condenser microphone with a main structure consisting of a movable film and an almost immobile backplate (capacitor plate). Two capacitor plates (electrodes) forming a capacitor may be made of a materiel of monocrystalline silicon, polycrystalline silicon, a composite layer consisting of polycrystalline silicon and silicon nitride, a composite layer consisting of silicon nitride and metal, etc., among which the monocrystalline silicon and polycrystalline silicon are two main solutions. When a sound pressure is applied to a film, the film has different displacements depending on the intensity of the sound pressure, resulting in a change in capacitance and accordingly in the output voltage, so that the corresponding sound signal can be read by means of an amplifying circuit.
According to the current main solutions, i.e. monocrystalline silicon and polycrystalline silicon, for the capacitor plate of MEMS microphone, the methods for forming an acoustic cavity thereby are different.
1. For the solution using pure monocrystalline silicon, an approach consists in forming a cavity by etching in a lower capacitor plate of the MEMS microphone, pre-embedding an etching stop layer on the back side and forming a CSOI material and completing the front side process before opening the acoustic cavity by etching from the back side to the pre-embedded etching stop layer on the back side. This solution has the disadvantages of very high requirements on the machining precision of monocrystalline silicon at the top layer and the uniformity of thickness, high costs of the existing mechanical grinding or SIMBOND process, a very low yield, and inability of mass production. Moreover, since the thickness of the lower plate is controlled by stopping the back side etching at the pre-embedded oxide layer, the overall process has a great discreteness.
2. For the solution using pure polycrystalline silicon, the acoustic cavity is a single cavity formed by one-time etching at the back side, the two polycrystalline silicon layers at the front side of the wafer are upper and lower plates of capacitor. However, during the manufacture, the polycrystalline silicon is processed for different lengths of time in the two steps, the film stress is influenced interactively by polycrystalline growth conditions and high temperature conditions, such as annealing, resulting in a very high difficulty in control.